The present invention relates to an optical encoder for detecting position, moving speed, moving direction and the like of a mobile unit by using a light-emitting element and a light-receiving element. In particular, the invention relates to an optical encoder which is suitable for use in printing equipment such as copiers and printers as well as in electronic equipment such as FA (Factory Automation) equipment.
As an optical encoder of this type, there has been provided one having such a positional relation between a mobile unit and photodiodes as shown in FIG. 4 (see Patent Document 1 (JP 2001-99684 A)). This optical encoder has a plurality of sets of three slits 2a, 2b and 2c and four photodiodes 3a, 3b, 3c and 3d, wherein the four photodiodes 3a-3d are placed so as to sequentially face the three slits 2a-2c formed in the mobile unit 1, in order to read four optical modulation signals.
More specifically, the plurality of slits 2a, 2b and 2c are bored in the mobile unit 1 at an array pitch P. Further, the four photodiodes 3a-3d each having a width of (½)P are positioned at a pitch of (¾)P (an electrical angle of 270°). Thus, an interval of (¼)P is provided between the neighboring photodiodes 3a-3d. When the mobile unit 1 moves relative to a light-emitting part (not shown) and the photodiodes 3a-3d, the photodiodes 3a-3d receive light which is derived from the light-emitting part and which has passed through the slits 2a, 2b and 2c, and independent four optical modulation signals (i.e., move information signals) A+, B+, A−, B− are outputted from the four photodiodes 3a-3d. 
In this case, for reading of move information of the mobile unit 1 by the optical encoder, light to be inputted onto the photodiodes 3a-3d from the light source of the light-emitting part ideally has a light quantity distribution evenly equal among light-receiving surfaces of the photodiodes 3a-3d. In such a case, it becomes possible to read only the move information associated with the mobile unit 1 accurately as an optical modulation signal.
However, there are involved, actually, dispersions of the light quantity distribution of the light source itself, dispersions of the light quantity distribution due to a focusing lens for the light source, the presence of diffracted light, refracted light and reflected light due to the mobile unit 1 or the like, variations in positional relations including the parallelism between the mobile unit 1 and the light-receiving surfaces of the individual photodiodes 3a-3d, size variations of the slits 2a-2c, variations due to contamination or the like of the light source, the mobile unit 1 and the photodiodes 3a-3d, variations in light reception sensitivity with respect to changes in the moving speed of the mobile unit 1, and so on. As a result of this, a plurality of move information signals acquired from the photodiodes 3a-3d have DC voltage offsets among signals, amplitude variations among signals, waveform distortions, phase variations among signals, and the like. This poses a problem that the acquired move information becomes inaccurate.
Thus, there has been proposed an optical encoder which has such a positional relation as shown in FIG. 5 between the mobile unit and the photodiodes in order to enhance the detection accuracy of the position, moving speed, moving direction and the like of the mobile unit 1 by Patent Document 1.
Hereinafter, each of the slits 2a-2c of FIG. 4 is generally designated as the slit 2 in some cases. Each of light-shielding parts 4a, 4b, 4c of the mobile unit 1 is generally designated as the light-shielding part 4 in some cases. Each of the photodiodes 3a-3d is generally designated as the photodiode 3 in some cases. Also, each of the slits 12a-12c of FIG. 5 is generally designated as the slit 12 in some cases. Each of light-shielding parts 14a, 14b, 14c of the mobile unit 11 is generally designated as the light-shielding part 14 in some cases. Each of the photodiodes 13a1, 13a2, 13a3, 13a4, 13b1, 13b2, 13b3, 13b4, 13c1, 13c2, 13c3, 13c4 is generally designated as the photodiode 13 in some cases.
This optical encoder includes a placement of k photodiodes 13 each of which results from equally dividing the photodiode 3 of Patent Document 1 by a number of x corresponding to k/n, which is a value resulting from dividing the common multiple k of m and n by n, where m (m is a natural number) is the number of slits 12 of a mobile unit 11, and n (n is a natural number) is the number of the move information signals. Then, output terminals of the photodiodes 13 are so connected every x terminals (x is a natural number) that the k photodiodes 13 output n independent move information signals.
For example, if m=3, n=4 and k=12, then the number x by which a widthwise length of the photodiode 3 i.e. a length of the photodiode 3 in the traveling direction of the mobile unit of Patent Document 1 is equally divided results in x=3. Thus, the light-receiving part composed of a plurality of photodiodes 13 is made up by arraying twelve photodiodes 13 each of which results from equally dividing the photodiode 3 of Patent Document 1 by three. Accordingly, the length of the mobile unit 11 on the light-receiving surface of the photodiode 13 in the traveling direction is one third of the length of the light-receiving surface of the photodiode 3 of Patent Document 1. In addition, output terminals of three photodiodes 13a1, 13b1, 13c1 are connected in common, and output terminals of three photodiodes 13a2, 13b2, 13c2 are connected in common. Also, output terminals of three photodiodes 13a3, 13b3, 13c3 are connected in common, and output terminals of three photodiodes 13a4, 13b4, 13c4 are connected in common. By these connections, the light-receiving part outputs the move information signals A+, B+, A−, B−, which are four independent high-accuracy optical modulation signals.
As shown above, with the optical encoder shown in FIG. 5, it becomes possible to reduce the light reception area of each photodiode 13 by subdividing each photodiode 3 as compared with the optical encoder of Patent Document 1. Therefore, shifts of move information signals obtained from the light-receiving part, waveform distortions, phase variations and the like can be suppressed, the resolution can be enhanced, and accurate move information can be obtained.
FIG. 6 shows a measurement result of photosensitivity with the use of the optical encoder shown in FIG. 5. According to FIG. 6, it can be understood that phase difference, amplitude and offset were improved over a measurement result of photosensitivity with the use of the optical encoder of Patent Document 1 shown in FIG. 7.
As shown above, with the optical encoder shown in FIG. 5, it becomes possible to improve the phase difference, amplitude and offset. However, the optical encoder shown in FIG. 5 has not completely eliminated the offset as can be seen from FIG. 6, nor having enough S/N (Signal-to-Noise) ratio. As a result of this, there is an issue that an optical encoder having even higher accuracy is necessary.
Further, with the use of a light source having larger dispersions of light quantity distribution for lower prices of the optical encoder, it is natural that larger dispersions on the light-receiving part side be involved. Due to this, when a small-size optical encoder is fabricated, enough lens condensation cannot be obtained, resulting in reception of light other than parallel light. Furthermore, when the slits 12 of the mobile unit 11 have a larger width, wraparound of light also becomes larger, making it impossible to obtain enough resolution. For this reason, a photodiode which is limitlessly strong to dispersions is needed.